A transmission-purpose power amplifier used for a wireless communication device particularly, from among other constituent parts of the wireless communication device, consumes power. Accordingly, the significant challenge in developing wireless communication devices is to improve the power efficiency of a power amplifier (PA). In recent years, the dominating communication standards are linear modulation for the purpose of improving spectral efficiency. This linear modulation has small tolerance for signal distortion.
Accordingly, in order to maintain the linearity, the average output power is set such that the instantaneous maximum output (peak) power becomes equal to or smaller than the saturation output of a power amplifier (PA). That is, as the ratio between the peak power and the average power in an amplified signal (Peak-to-Average Ratio, hereinafter abbreviated as PAR) increases, the average output power must be set to a value much lower than the saturation output of the power amplifier (PA) in order to maintain the linearity.
However, a power amplifier (PA) in general exhibits the following characteristic: as the average output power is reduced to assume a lower proportion relative to the saturation output power, the ratio between the DC supply power of the power amplifier (PA) and the obtained transmission power (the power efficiency) is reduced. A reduction in the power efficiency hinders energy saving.
The PAR of a communication signal has a unique value for each communication standard. With recent high speed wireless communications such as CDMA (Code Division Multiple Access), WLAN (Wireless Local Area Network), digital terrestrial television broadcasting, and LTE (Long Term Evolution), the PAR assumes a great value of about a few dB to ten-odd dB. Such a great PAR causes a great reduction in the power efficiency of the power amplifier (PA).
As means for solving the problem of a reduction in the power efficiency attributed to the great PAR, there is a method in which a communication signal is subjected to processing for reducing the PAR, and thereafter the communication signal is input to a power amplifier (PA).
An exemplary scheme for reducing the PAR of a communication signal is disclosed in Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2000-138645 “Multi-Carrier Transmission Circuit and Communication Device”. FIG. 37 is a block configuration diagram showing the block configuration of a transmission apparatus disclosed in Patent Literature 1, in which a structure for reducing the PAR of a communication signal is shown. Patent Literature 1 particularly provides a scheme for reducing the PAR of a multi-carrier signal intended for use in the CDMA technique. In FIG. 37, channel signals of respective carriers are input to input terminals 1-1, 1-2, . . . , 1-n, respectively. The channel signals are upconverted to respective carrier frequencies by modulators 5-1, 5-2, . . . , and 5-n. Thereafter, the signals are combined at an adding circuit 6 and output as a multi-carrier signal from the output terminal 8. The multi-carrier signal is input from the output terminal 8 to a power amplifier (PA: not shown in FIG. 37).
In connection with the multi-carrier signal, the peak is enhanced when the carrier signals are in phase, whereas the peak is reduced when the carrier signals are in opposite phase. Accordingly, with the transmission apparatus shown in FIG. 37, the phase of each of the carrier signals is detected by phase detectors 4-1, 4-2, . . . , 4-n, and output to a control circuit 7. Thus, in the situation where the carrier signals are in phase, the control circuit 7 controls variable attenuators 2-1, 2-2, . . . , 2-n such that the variable attenuators 2-1, 2-2, . . . , 2-n attenuate the channel signals. Thus, a reduction in the PAR of the combined multi-carrier signal is realized. In this manner, with the transmission apparatus disclosed in Patent Literature 1, as the phase information is acquired, an accurate PAR reduction can be achieved.
Similarly to Patent Literature 1, an exemplary scheme for reducing the PAR of a communication signal by controlling the phase of each carrier signal is disclosed in Patent Literature 2: Japanese Unexamined Patent Application Publication No. H05-130191 “Method for Reducing Peak/Average Value Ratio by Controlling Phase of Multi-Sub-Channel Signal”. FIG. 38 is a block configuration diagram showing the block configuration of a transmission apparatus disclosed in Patent Literature 2. Similarly to Patent Literature 1, in Patent Literature 2 the structure for reducing the PAR of a communication signal is shown.
In the structure of a transmission apparatus 80 disclosed in Patent Literature 2 shown in FIG. 38, serial data input to a serial/M parallel converter 51 is converted into data signals of lower speeds, and output to 16 QAM/symbol converters 52. The data signals output from the serial/M parallel converters 51 pass through 16 QAM/symbol converters 52, pilot symbol inserters 53, low-pass filters 54, quadrature modulators 56, and phase shifters 86, and combined at a multiplexer 57, to be output to a power amplifier 58.
In the transmission apparatus 8 shown in FIG. 38, when the data signals respectively output from the phase shifters 86 are in phase, the peak value of the data signal combined at the multiplexer 57 becomes the maximum. Accordingly, in order to prevent the data signals respectively output from the phase shifters 86 from becoming in-phase, the phase of each of the data signals is adjusted at each of the phase shifters 86. Thus, the peak value of the data signal combined at the multiplexer 57 can be reduced.
Further, Patent Literature 3: Japanese Patent No. 3714917 “Peak Limiter and Multi-Carrier Amplifier Apparatus” discloses a PAR reduction scheme in which a simplified circuit structure is achieved in exchange for reduced accuracy, which is attributed to lack of acquisition of phase information. FIG. 39 is a block configuration diagram showing the block configuration of a transmission apparatus disclosed in Patent Literature 3, in which, similar to Patent Literature 1, the structure for reducing the PAR of a communication signal is shown. Patent Literature 3 particularly provides a scheme for reducing the PAR of a multi-carrier signal intended for use in the CDMA technique.
An instantaneous amplitude value a1(t) of an input signal 1 (I1(t), Q1(t)) is generally given by:a1(t)=sqrt[{I1(t)}2+{Q1(t)}2]
Here, sqrt [ ] is a function providing a square root. The instantaneous power P1(t) of the input signal 1 is proportional to {a1(t)}2. Further, an instantaneous amplitude value a2(t) of an input signal 2 (I2(t), Q2(t)) is given by the following, similarly to the input signal 1:a2(t)=sqrt [I2(t)}2+{Q2(t)}2]
The instantaneous power P2(t) of the input signal 2 is proportional to {a2(t)}2.
In the peak limiter 11 shown in FIG. 39, an instantaneous power detecting unit 12 detects, as the approximating instantaneous power of a multi-carrier signal, combined power of the input signal 1 and the input signal 2, which is P(t)=P1(t)+P2(t)∝{a1(t)}2+{a2(t)}2. In practice, accurate instantaneous power of the multi-carrier signal is, as mentioned in Patent Literature 1, depends on not only the amplitudes a1(t) and a2(t) but also phase θ1(t)=arctan(Q1(t)/I1(t)) of the input signal 1 and phase θ2(t)=arctan(Q2(t)/I2(t)) of the input signal 2.
However, in Patent Literature 3, since the instantaneous power is approximately derived solely by the amplitudes a1(t) and a2(t), the phase detector is omitted, whereby the simplified circuit is achieved. Further, when the instantaneous power detected by the instantaneous power detecting unit 12, i.e., P(t)=P1(t)+P2(t)∝{a1(t)}2+{a2(t)}2, exceeds a predetermined peak threshold value, a limiter unit 14 suppresses the signal amplitudes, to thereby reduce the PAR of the signal. The signal whose PAR is reduced is output as an output signal 21 and an output signal 22. The output signal 21 and the output signal 22 are up-converted to the carrier frequencies by a modulator (not shown in FIG. 39), and input to a power amplifier (PA: not shown in FIG. 39).
Similarly to Patent Literature 3, Patent Literature 4: Japanese Patent No. 4354649 “Time Offset Technique for Increasing the Capacity of a CDMA System” also discloses a PAR reduction scheme in which a simplified circuit structure is achieved in exchange for reduced accuracy, which is attributed to lack of acquisition of phase information. Patent Literature 4 also provides a scheme for reducing the PAR of a multi-carrier signal particularly intended for use in the CDMA technique. FIG. 40 is a schematic diagram showing the concept of the scheme for reducing the PAR of a communication signal disclosed in Patent Literature 4. FIG. 40 shows transmission waveforms of a CDMA system 70, in which a first transmission waveform 74A and a second transmission waveform 74B are transmitted together.
In the CDMA system 70, pilot portions 78 have particularly high signal power. In the case where the first transmission waveform 74A and the second transmission waveform 74B are transmitted at the same timing, the pilot portion 78 which is the high power portion of the first transmission waveform 74A and that of the second transmission waveform 74B overlap each other at the same timing. Accordingly, the peak of the signal power may be disadvantageously increased.
Accordingly, as shown in FIG. 40, allowing the send-out timing of the second transmission waveform 74B to be displaced by a time-offset of t0, the pilot portion 78 which is the high power portion of the first transmission waveform 74A and that of the second transmission waveform 74B are prevented from temporarily overlapping each other, whereby the peak of the signal power is reduced. Note that, in Patent Literature 4, similarly to Patent Literature 3, the peak transmission power is also estimated by the sum of power of the transmission waveforms.
Further, similarly to Patent Literature 3, a PAR reduction scheme in which no phase information is acquired is disclosed in Patent Literature 5: Japanese Unexamined Patent Application Publication No. 2002-305489 “Code Multiplex Signal Transmission Apparatus”. FIG. 41 is a circuit diagram showing a carrier multiplexing circuit that reduces the PAR of a communication signal disclosed in Patent Literature 5.
A carrier multiplexing circuit 50 shown in FIG. 41 includes code multiplex signal transmitting units 511 to 51n and an adding unit 52a. The code multiplex signal transmitting units 511 to 51n respectively output RF signals on respective carrier frequencies to the adding unit 52a. The adding unit 52a combines the RF signals on the respective carrier frequencies and input the combined signal to the power amplifier 53a. 
Note that the code multiplex signal transmitting units 511 to 51n each include a code multiplex signal generating unit 61, a peak suppressing unit 62, a delay unit 63, a filter 64, a frequency shifting unit 65, and a transmission circuit 66.
In the carrier multiplexing circuit 50, the amplitudes of input/output signals x1 to xn of the delay units 63 in the respective code multiplex signal transmitting units 511 to 51n are sensed and compared by a comparator 56a2 in an amplitude control unit 56a, and a selector 56a3 in the amplitude control unit 56a selects a signal having the great amplitude peak out of the signals x1 to xn. The signal selected by the selector 56a3 is multiplied by a suppression coefficient by multiplier units 56a11 to 56a1n. By the multiplication by the suppression coefficient, out of the RF signals output from the code multiplex signal transmitting units 511 to 51n, the RF signal with the great amplitude peak has its amplitude suppressed. Accordingly, the amplitude peak value of the combined RF signal output from the adding unit 52a is also suppressed.
In the foregoing, the conventional techniques for reducing the PAR of a communication signal are summarized. The conventional techniques for reducing the PAR of a communication signal as described above are intended to be used in the situation where frequencies of respective carrier signals are close to one another, such as the multi-carrier CDMA communication scheme, i.e., the situation where the frequency difference Δf between the carrier frequencies and the modulation bandwidth fBB of the carrier signals become substantially equivalent (Δf≅fBB).
On the other hand, as disclosed in Non-Patent Literature 1: Nobuhiko Miki et al. “Carrier Aggregation Realizing Increased Bandwidth in LTE-Advanced”, the Carrier Aggregation (CA) technique in which a plurality of band fragments are used as gathered is employed in recent communication standards, in order to realize wireless communications of higher speeds. The CA technique enables a plurality of bands to be bundled to secure a wide band, to thereby increase the transmission rates.
Further, in the Inter-band Non-contiguous CA mode (Δf>>fBB) in which the carrier frequencies are fully separated from one another, the communication stability can be improved by establishing simultaneous communications at a plurality of carrier frequencies differing in the propagation characteristic. Further, application of the CA technique enables communications supporting the case where the band allocation among a plurality of service providers is intermittent, or where a band is shared.
FIG. 42 is a block configuration diagram showing the block configuration of a transmitter disclosed in Patent Literature 6: Japanese Unexamined Patent Application Publication No. 2004-289428 “Multiband Power Amplifier Module”. In the transmitter shown in FIG. 42, a power amplifier (PA) 311, a power amplifier 321, and a power amplifier 331 amplify and output a communication system signal on a carrier frequency f1, whereas a power amplifier PA 312, a power amplifier 322 and a power amplifier 332 amplify and output a communication system signal on a carrier frequency f2. The output signals from the power amplifier 331 and the power amplifier 332 are combined at matching circuits 251 and 252 and a multiplexer circuit 60, and the combined signal is output to an output terminal 41.
FIG. 43 shows an explanatory diagram that describes the transfer function from an output terminal 51b of the power amplifier 331 in the transmitter shown in FIG. 42 to the output terminal 41 and the transfer function from an output terminal 52b of the power amplifier 332 to the output terminal 41. As shown in FIG. 43, only the RF signal on the frequency f1 is transmitted from the output terminal 51b to the output terminal 41, and only the RF signal on the frequency f2 is transmitted from the output terminal 52b to the output terminal 41. Accordingly, the RF signal on the frequency f1 output from the power amplifier 331 will not establish a sneak path to the power amplifier 332. Also, the RF signal on the frequency f2 output from the power amplifier 332 will not establish a sneak path to the power amplifier 331.
Thus, the power loss attributed to any sneak path of the RF signals is suppressed. In the transmitter of the conventional technique disclosed in Patent Literature 6, since the RF signals differing in frequency are individually amplified and output by the two power amplifiers 331 and 332, two RF signals differing in frequency can be simultaneously transmitted.